Amplitude modulated vestigial side band cable TV (CATV) systems operating at a wavelength of 1.55 um offer advantages over those at 1.3 um because of low fiber loss and availability of erbium-doped fiber amplifiers. For long link or trunk link applications at 1.55 um however, directly modulated transmitters are not normally used because the interaction laser chirp and fiber dispersion generates an intolerable signal interference. Laser chirp also induces some noise because of double Rayleigh backscattering (DRB), which becomes more detrimental as transmission link lengthens. Although dispersion compensation modules (DCM) can be used to compensate for the fiber dispersion, use of DCM adds the inconvenience of network design, insertion loss and cost. As a result, transmitters based on lithium niobate Mach-Zehnder external modulators (LN EM) are used because of very low chirp in amplitude modulated vestigial side band commercially. LN EM offers high performance at premium cost and complexity in manufacturing. LN EM are also widely used in digital applications also for their low chirp.
Integrated electro-absorption modulated lasers (EML) also have low chirp and they are compact and much cheaper when compared to LN EM. However EMLs are not widely used in analog CATV applications due to the fact that they are usually not linear enough for analog video signal modulation.
What is needed is a proper distortion correction technique, so EML components can generate acceptable distortion performance for typical analog applications. This allows analog CATV transmitter design with a more compact and much lower cost for trunk transmissions at a wavelength of 1.55 um.
Because the laser diode and electro-optic modulator are built on the same substrate for an EML, they can be packaged in a small form factor. As a result, there may exist leakage between the laser diode and the electro-optic modulator, when either the dithering signal or the electro-optic modulator signal or both are modulated.
When the electro-optic modulator is an electro-absorptive (EA) modulator, leakage from laser diode and the electro-optic modulator can also be generated through EA absorption because the absorbed light at the EA is converted into photocurrent. The photocurrent can in turn affect the EA bias. The dithering bearing light is therefore “leaked” to the EA modulator.
Typically, EMLs are operated to transmit a digital information signal, meaning that the information input to the EML for transmission is input in one of two states, a low state and a high state. Digital input allows for robust performance because noise in the system must be above a threshold value to have an impact on the system. The robust noise performance allows EMLs to transmit a digital information signal with little effect from any mutual leakage in the input signals.
When an EML transmits an analog information signal, signal interference resulting from leakage may be large enough to distort the output of the EML making an EML less useful in such a situation.
What is needed is system and method for reducing or cancelling mutual leakage in an electro-absorption modulated laser generated from input signals.